Preparation of sulfate pulping liquor with polysulfide



NOV. 9, 1965 p LANDMARK 3,216,887

PREPARATION OF SULFATE PULPING LIQUOR WITH POLYSULFIDE Filed May 18, 1964 AGITATED GAS L\QU\D QONTACTOR OXYGEN- METER M SPENT AHZ C 1 i D I $\EVE PLATE -APPARATUS o o O O O O Q o L A .J O O O O o o o O O O O O o c V o o O O ROTAMETER O o O O JTPERFORATED l STEAM E| PLATE Fk-Am AIQ INVENTOR: PER ANDREAS LANDMARK Hi5 ATTORNEY verted into sodium hydroxide by causticizing.

United States Patent 3,216,887 PREPARATION OF SULFATE PULPING LIQUOR WITH POLYSULFHDE Per Andreas Landmark, Oslo, Norway, assignor to Papirindustriens Forslrningsinstitutt, Oslo, Norway Filed May 18, 1964, Ser. No. 368,365 Claims priority, application Norway, May 27, 1961, 140,345 7 Claims. (Cl. 16238) This application is a continuation-in-part of my application Serial No. 184,663, filed April 3, 1962, now abandoned, entitled Improvements in Sulphate Pulping Processes.

This invention relates to a method for preparing a cooking liquor with a high content of polysulphide to be used in the sulphate wood pulping process. The high content of polysulphide is intended to increase the pulp yield and to prevent corrosion of the digesters.

In the sulphate process the wood is digested in an alkaline cooking liquor. The active components in the liquor are sodium hydroxide and sodium sulphide. The process cannot be carried out in an economical manner Without recovery of said chemicals. The spent liquor which is called black liquor is, therefore, evaporated and combusted. The melt of inorganic chemicals obtained as a residue from this combustion is dissolved and form the green liquor essentially consisting of sodium carbonate and sodium sulphide. The sodium carbonate is then $11:-

e causticized clear liquor is called white liquor. In the process some chemicals are lost and said chemicals are usually compensated by addition of sodium sulphate to the combustion oven.

The eiiective alkali in the liquor is defined as the sum of the content of sodium hydroxide and half of the content of sodium sulphide calculated as sodium hydroxide (NaOH+ /2Na S). The sulphidity of the liquor is defined as the ratio between the content of sodium sulphide and the sum of sodium hydroxide and sodium sulphide, calculated as Na-equivalents Nags By charging the digesters, White liquor and black liquor are added until the desired amount of effective alkali and the desired ratio between liquor and wood is obtained.

In the conventional process for the preparation of un bleached sulphate pulp the degree of digestion can be varied only by varying the amount of effective alkali or the length of the digesting period. In this way it is not possible to obtain any essential increase in the yield of the carbohydrates per se, which is of importance in the production of both bleached and unbleached pulp. The reason for this is that during the last stage of the digesting practically only lignin is dissolved. The major part of the carbohydrates which is dissolved during the digesting has at that time already gone into solution.

It is, however, prior known that considerable increases in the carbohydrate yield can be obtained by addition of special chemicals to the cooking liquor, such as sodium boro-hydride and sodium polysulphide. Both act as stabilizing agents on the alkali labile groups in the carbohydrates, these groups being responsible for the degradation of the carbohydrates. According to U.S. Patent No. 2,944,928, FIG. 2, the total yield calculated on wood increases from 47% to about 51% when the content of polysulphide sulphur in the cooking liquor is increased from about 0% to 4.5% calculated on wood.

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The content of polysulphide in the cooking liquor is also of importance for the corrosion of the digester. Corrosion of sulphate digesters made of mild steel plates is caused by the polysulphide in the cooking liquor, this chemical acting as a cathodic depolarizor. When the steel plates are active the rate of corrosion increases with increasing concentrations of Na S NaOI-I, Na s and Na S O in the cooking liquor, while increasing content of Na SO will reduce the rate of corrosion. However, corrosion is critical only during the first stage of the cooking period. The metal later becomes passive, whereby further attack on the metal is prevented. This has been established by weight loss experiments (L. Stockman and L. Ruus, Vvensk Papperstidning 57, 831 (1954)), and by electrochemical measurements carried out by use of a specially developed Ag/AgS electrode.

The tendency of the steel plates to become passive is strongly influenced by the composition of the cooking liquor, but not in the same way as the rate of corrosion of the active steel plates. While the rate of corrosion increases With increasing amounts of polysulphide when the content of the same is low, a high content of polysulphide will induce passivation.

Electrochemical measurements have shown that the passivation tendency decreases with increasing content of Na S and NaOH in the cooking liquor, while an increase of the Na S content strongly increases the passivation tendency. The action of Na s is considerably greater than the activating action of Na S and NaOH.

Because of this it is possible to reduce the corrosion in the digesters by preparing a cooking liquor which increases the tendency of the plates to become passive, thus eliminating the critical period.

A polysulphide enriched cooking liquor can be prepared by adding alkali polysulphide to the cooking liquor. This method is disclosed in the cited US. Patent No. 2,944,928. As prior known, polysulphide enriched cooking liquor can also be prepared by dissolving elemental sulphur in an aqueous sodium hydroxide or sulphide solution. Said method is used, for example, by Peckham and May: TAPPI (January 1960), No. 1: 45. However, the use of cooking liquors prepared according to these methods give undesirable effects in the liquor recovery system. The polysulphide added, whether in the form of polysulphide as such or in the form of elemental sulphur, will after digesting and the subsequent combustion of the black liquor occur as sulphide and in amounts eX- ceeding the normal sulphur losses in the process. This causes a continuous increase in the sulphidity of the cooking liquor, resulting in an increased risk of a more poorly digested pulp, and an increased corrosion in the recovery system as well as increased contamination. This is well known in the field, and efforts have been made for eliminating these obstacles. The process which will be described in the following significantly reduces these difficulties.

It is prior known that polysulphide can be formed by oxidation with air of neutral sodium hydrogen sulphide solutions (see US. Patent No. 2,135,879). White liquor 1s, however, strongly alkaline, and its sulphide is not easily oxidized by air. Moreover, oxidation of white liquor will not give sodium polysulphide but sodium thiosulphate.

The present invention is based upon the fact that by oxidation of a mixture of black liquor and white liquor the lignin dissolved in the black liquor, the sulphate llgnin, will cause formation of considerable amounts of polysulphide together with thiosulphate, and the oxidation also proceeds faster.

W. A. Mueller (Pulp and Paper Magazine of Canada 66T3 (1959)) points out that the tendency of passivation o of the digester plates increases when the digesters are charged with oxidized black liquor. W. A. Mueller then refers to a black liquor which is oxidized under conditions which aim at the conversion of Na S to Na S O Said oxidation is carried out in a number of sulphate mills, primarily in order to reduce evaporator corrosion.

The process according to the present invention is based on the discovery that when the mixture of black liquor and white liquor, which is charged to the digester, is partially oxidized with air in a suitable equipment, polysulphide is formed, which in turn increases the pulp yield and passivates the steel plates of the digesters.

Said oxidation process differs significantly from the conventional oxidation of black liquor in that the latter aims at a complete conversion of Na S into Na S O whereas the present invention prescribes a partial conversion of Na s to Na s in a mixture of black liquor and white liquor, while further oxidation to Na S O is mainly avoided. Polysulphide concentration high enough always to secure passivation of the plates cannot be obtained by oxidation of black liquor alone.

Thus the said partially oxidized mixture is a new product which is fundamentally different from the industrial produced called oxidized black liquor.

Below an embodiment of the invention will be described as an example:

(1) One starts with a white liquor with high sulphidity (40%100%) which may be higher than is usual today.

(2) To the white liquor black liquor is added. Nonevaporated black liquor (raw liquor), partly evaporated black liquor (weak liquor), or evaporated black liquor (strong liquor) can be used. The black liquor content of the mixture can vary between and 80%.

(3) The mixture is oxidized with air in a suitable equipment. Air of atmospheric pressure or compressed air can be used. During the oxidation the sodium sul phide content of the liquor will decrease and the polysulphide content will increase to a maximum. By further oxidation the polysulphide content will again decrease and the initial sulphide content will finally be totally converted into thiosulphate. The oxidation must, therefore, be interrupted at a time when the polysulphide content reaches its highest value. This will usually occur when 1435 g. Na S/l. is oxidized. The required amount of air for such an oxidation is by complete utilization of the air oxygen about 20-50 times the liquor volume. By practical embodiment of the oxidation there will often be desirable to use considerably higher amounts of air (100-1000 times the liquor volume). Thus there is here an intended oxidation in a suitable equipment. Polysulphide amounts suflicient for the increase of yield will not be formed by an incidental contact between air and liquor.

(4) The digester is charged with the oxidized mixture of white and black liquor and the liquor to wood ratio is regulated by addition of small amounts of black liquor. The digestion is carried out in the usual way and the chemicals are recovered as described for the conventional sulphate process. As the process presupposes a high sulphide content in the liquors, the sulphur losses will be higher than normal during the recovery process. The increased sulphur losses can, therefore, best becompensated by addition of elemental sulphur which. is dissolved in the cooking liquor.

Below some examples of the process as carried out in the laboratory shall be referred:

(1) The oxidation was carried out in an apparatus at 70 C. as shown in FIG. 1 (agitated gas-liquid contactor). The rate of air was determined by means of a rotameter. The air was supplied to the bottom of the tank and was distributed in the liquor by means of a rotating agitator and baffles inserted in the walls of the tank. Foam was depressed by means of fan blades mounted on the agitators axis. The spent gas is led through an instrument for measurement of the residual oxygen content. The composition of the liquor before the oxidation was:

Ratio white liquor:black liquor 1.6:1 Effective alkali -g./l 68 N32820:; g./l Na S (calculated as Na S g./l 2 Liquor volume in the apparatus l 2.4 .Rate of air flow l./h 300 Residual 0 content in the spent gas ..percent 4 Oxidation time mins 25 Ratio airzliquor 52:1

Composition of the oxidized liquor:

Effective alkali g./l 74 Na S g./l 37 Nagsgog g./l Na S (calculated as Na S g./l 25 By experimental cooking with laboratory chips from spruce with effective alkali 18.2% NaOH (on wood) in said oxidized liquor, 2.5% higher pulp yield (on wood) at Kappa No. 35 was obtained than by conventional sulphate digesting with effective alkali=18.2% NaOH (on wood) under otherwise equal cooking conditions.

(2) The oxidation was carried out at 70 C. in an apparatus as shown in FIG. 2 (sieve plate apparatus).

The air was fed into the apparatus below the sieve bottom.

To prevent the temperature of the liquor to decrease during the oxidation, the air was preheated to about 70 C. and saturated with water vapor before it entered the sieve bottom. The total hole area of the sieve bottom was 3.3% of the total area and the diameter of the holes'was 2.5 mm.

The composition of the liquor before the oxidation was:

Ratio white liquorzblack liquor 1.6:1 Effective alkali g./l 70 Na S g./l 46 Nazszog g./1 Na s (calculated as Na S g./l 2 Liquor volume in the apparatus 11.5 Rate of air flow 1./s 16 Oxidation time mins 5 Ratiod airzliquor 320021 The composition of the oxidized liquor was:

Efiective alkali g./l 73 Na S g./l 25 Nazszo3 g./l Na s (calculated as Na S g./l 17 To the oxidized liquor was added 1% S (on wood), and digesting experiments were carried out on laboratory chips of spruce with an amount of liquor corresponding to an effective alkali=18.5% NaOH (on wood).

The yield of pulp (on wood) was at Kappa No. 35, 3.4% higher than by conventional sulphate digestion and 2.4% higher than from digesting experiments with nonoxidized cooking liquor to which 1% S (on wood) was added, under less equal cooking conditions.

I claim:

1. In a process, for the preparation of a cooking liquor of high polysulfide content, the steps comprising, digesting cellulose material in a sulfate pulping process, thereafter separating the spent, black liquor from the digested material, said black liquor containing dissolved sulfate lignin, subsequently mixing of from 5 to percent by volume of black liquor with from 20 to percent by volume of white liquor, said white liquor, having a sulfidity of from 40 to percent, thereafter partially oxidizing the mixture with oxygen with a consumption of from 1.4 to 14 cubic meters of oxygen per cubic meter of said mixture, said dissolved sulfate lignin promoting the formation of polysulfide, whereby the sodium sulfide content of the mixture will be decreased and the polysulfide content of the mixture will be increased.

2. In a process, as claimed in claim 1, said black liquor being non-evaporated raw black liquor.

3. In a process, as claimed in claim 1, said black liquor being partly evaporated concentrated weak black liquor.

4. In a process, as claimed in claim 1, said black liquor being evaporated strong black liquor.

5. In a process, as claimed in claim 1, the amount of polysulfide, calculated as Na S of the mixture prior to oxidation being about 2 grams per liter, and after the oxidation being about grams per liter.

6. A process according to claim 1, in which the oxygen is supplied as air.

7. A process according to claim 1, in which the content of white liquor in said mixture of black and white liquor is from about to about References Cited by the Examiner UNITED STATES PATENTS 9/49 Fuller 16238 7/60 'Kibrick 162-82 OTHER REFERENCES DONALL H. SYLVESTER, Primary Examiner. 

1. IN A PROCESS, FOR THE PREPARATION OF A COOKING LIQUOR OF HIGH POLYSULFIDE CONTENT, THE STEPS COMPRISING, DIGESTING CELLULOSE MATERIAL IN A SULFATE PULPING PROCESS, THEREAFTER SEPARATING THE SPENT, BLACK LIQUOR FROM THE DIGESTED MATERIAL, SAID BLACK LIQUOR CONTAINING DISSOLVED SULFATE LIGNIN, SUBSEQUENTLY MIXING OF FROM 5 TO 80 PERCENT BY VOLUME OF BLACK LIQUOR WITH FROM 20 TO 95 PERCENT BY VOLUME OF WHITE LIQUOR, SAID WHITE LIQUOR, HAVING A SULFIDITY OF FROM 40 TO 100 PERCENT, THEREAFTER PARTIALLY OXIDIZING THE MIXTURE WITH OXYGEN WITH A CONSUMPTION OF FROM 1.4 TO 14 CUBIC METERS OF OXYGEN PER CUBIC METER OF SAID MIXTURE, SAID DISSOLVED SULFATE LIGNIN PROMOTING THE FORMATION OF POLYSULFIDE, WHEREBY THE SODIUM SULFIDE CONTENT OF THE MIXTURE WILL BE DECREASED AND THE POLYSULFIDE CONTENT OF THE MIXTURE WILL BE INCREASED. 